chest x-ray manifestations of pneumonia
TRANSCRIPT
Chest X-ray manifestationsof pneumoniaDebasis Das
David C Howlett
Abstract Pneumonia is a leading cause of morbidity and mortality in the UK, and
chest X-rays are the initial modality of investigation in most cases.
Broad categories of infective change can be recognized on chest X-ray,
and are associated with different aetiological organisms. These chest
X-ray patterns, including lobar pneumonia, bronchopneumonia, nodular
consolidation, interstitial consolidation, atypical pneumonia, and lung
abscesses, are reviewed and the commonest micro-organisms that are
likely to be responsible discussed. The sequelae of pneumonia, and the
differential diagnoses that pneumonia is frequently mistaken for are
also discussed. Knowledge of the chest X-ray manifestations of
pneumonia will help readers to guide appropriate therapy in the future,
and anticipate any complications that may arise.
Keywords consolidation; chest X-ray (CXR); pneumonia
Pneumonia can be defined as a respiratory infection that
produces consolidation of the lungs. It is one of the commonest
causes of mortality and morbidity within the UK, and chest X-ray
(CXR) represents the initial investigation of choice in most cases.
A working knowledge of the appearances of pneumonia on CXR
is therefore, important for the successful management of this
commonly occurring condition.
What is consolidation?
Consolidation is essentially inflammatory exudate within the
lung tissue. This results in the normally lucent (black), air-filled
pulmonary tissue appearing opaque (white) on CXR. Different
infective organisms tend to produce consolidation with differing
distribution around the lung and, when taken together with
clinical information, recognition of these different ‘patterns’ of
infection on CXR can narrow down the list of likely causative
organisms and help guide appropriate therapy.
Patterns of consolidation on CXR
Air-space consolidation
The air spaces of the lungs comprise the alveoli, alveolar ducts,
and respiratory bronchioles. Infection that originates in the air
spaces produces ‘Lobar pneumonia’ (see Figure 1).1 This pattern
of consolidation is characterized by:
a solitary, peripheral focus of dense opacity
sparing of the larger airways to produce ‘air bronchograms’
(air-filled bronchi surrounded by opacity)
progressive spread to adjacent areas and eventually towards
the hilum
Lobar pneumonia is classically associated with community
acquired infection, and is most commonly due to Streptococcus
pneumoniae (Gram-positive bacterium).
Less commonly, community-acquired lobar pneumonia may
be caused by Legionella pneumoniae (Gram-negative
organism). The diagnosis is suggested by associated non-
respiratory signs and symptoms (e.g. diarrhoea, neurological
dysfunction, raised liver function tests).
Tuberculosis (TB) can also cause homogeneous consolidation
similar to lobar pneumonia. Upper and middle lobe prefer-
ence, associated enlarged lymph nodes, and a compatible
history (e.g. patient from Indian subcontinent, or immuno-
compromized) are all supportive.
Figure 1 Lobar pneumonia: homogeneous consolidation in the left upper
zone with air bronchograms (black arrows). Concomitant bronchopneu-
monia is also evident in the right mid zone e patchy consolidation
without air bronchograms (white arrows).
Learning points
C Recognition of different CXR patterns in respiratory sepsis can
help to narrow the list of differential causes
C Pneumonia is associated with complications such as pleural
effusion, abscess formation and pneumothorax e look out for
them!
C Alternative organisms, or non-infectious causes of consolida-
tion (including cancer), should be considered if there is limited
clinical improvement in the patient, or in the appearance of
the CXR following treatment.
Debasis Das MRCS(Eng) is a Specialist Registrar within the Department of
Radiology at Eastbourne District General Hospital, East Sussex, UK.
Conflicts of interest: none declared.
David C Howlett MRCP(UK) FRCR is a Consultant Radiologist within the
Department of Radiology at Eastbourne District General Hospital, East
Sussex, UK. Conflicts of interest: none declared.
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SURGERY 27:10 453 2009 Published by Elsevier Ltd.
Infection that originates in the airways and then spreads to the
air spaces, however, produces a slightly different pattern of
consolidation termed ‘Bronchopneumonia’ (see Figure 1).
Characteristic features include:
multiple areas of patchy consolidation, often bilaterally
lack of air bronchograms (as the airways are consolidated)
progressive coalescence of the patchy consolidation with time.
Bronchopneumonia is typically associated with hospital
acquired (nosocomial) organisms such as Escherichia coli and
Pseudomonas aeruginosa (Gram-negative bacteria). Community
acquired bronchopneumonia does occur and is classically caused
by Staphylococcus aureus (Gram-positive bacteria), though it too
frequently causes nosocomial infection as well. Remember:
Modern treatment regimens for nosocomial respiratory sepsis
cover both Gram-positive and Gram-negative bacteria, so
aetiological origin is not as important as simply identifying
the consolidation.2
It is important to understand that the patterns described above
detail the established changes of pneumonia on CXR. The
features of early infection may be more subtle, and looking for
the ‘silhouette sign’ may be useful in such circumstances:
normally distinct borders of opaque structures such as the heart,
aorta, and diaphragm appear unclear/irregular (commonly
referred to as ‘loss of the hemidiaphragm/heart border’)3
the appearance is caused by a small focus of consolidation
(which is also opaque) lying adjacent to these structures e
the similar densities of these tissues prevents a clear differ-
entiation between them on CXR.
Nodular consolidation
Nodules are small, rounded foci of air-space opacity, and are
usually associated with non-bacterial or uncommon bacterial
infections.1,2,4 Examples include:
Varicella zoster (chicken pox virus), which produces wide-
spread, bilateral nodular pneumonia (see Figure 2)
Mycoplasma pneumoniae (atypical bacterium), which usually
causes nodular pneumonia within one lung. These nodules
can coalesce as the infection progresses and produce a patchy
bronchopneumonia-type pattern (nodules and denser
consolidation may be seen together within one lung at this
stage).
TB also commonly produces nodular opacification, though
predominantly within the upper portions of the lungs.
Progression of infection can also lead to coalescence of the
nodules and a patchy, bronchopneumonia-type pattern.
Interstitial consolidation
As well as being divided into lobes and segments, the lungs are
further subdivided into millions of microscopic lobules which
contain the alveoli, or air spaces, described before. The lobules
are separated by interlobular septa, and the space between
individual lobules is called the interstitial space. Respiratory
infections do not commonly produce opacification of the inter-
stitial space, but when they do, the appearances are quite distinct
from the air-space opacification:1,2,4
numerous short, opaque lines (reticulations)
symmetrically distributed in both lungs
frequent association with small, opaque nodules, which often
requires close inspection of the CXR in order to differentiate
them from the reticulations (the CXR is described as having
a ‘reticulo-nodular’ pattern in these circumstances)
Organisms producing interstitial consolidation are usually associ-
ated with immunocompromized patients, including those with AIDS.
Pneumocystis carinii (protozoa) is a good example (see Figure 3).
Atypical pneumonia
A non-specific term that is used to describe:
pneumonias with common (lobar or bronchopneumonic)
CXR patterns but unusual clinical history, such as Legionella
infection (which can present with gastrointestinal and
neurological symptoms)
pneumonias with unusual CXR patterns, namely nodular and
interstitial consolidation, such as Mycoplasma and Pneumo-
cystis infections, respectively
Figure 2 Nodular consolidation in a patient with chicken pox: multiple
rounded opacities can be seen throughout both lungs.
Figure 3 Interstitial pneumonia secondary to Pneumocystis carinii :
bilateral and symmetrical opacities can be seen in the mid zones. Close
examination of the film will reveal the presence of innumerable, thin
opaque lines that are superimposed upon one another and deceptively
appear as homogeneous areas of consolidation.
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SURGERY 27:10 454 2009 Published by Elsevier Ltd.
The relevance of broadly categorising such pneumonias as
‘atypical’ is due to the fact that they frequently require non-
standard therapy, including uncommon antimicrobial agents.
Most hospitals have antibiotic prescription protocols in place
to manage such atypical infections empirically prior to
specific diagnosis.
Lung abscess
The difference between a lung abscess and consolidation is the
presence of an epithelial wall around the former. This is a histo-
pathological observation and is not perceptible on CXR, resulting
in most lung abscesses appearing identical to focal areas of
consolidation.1 Unlike regular consolidation, however, abscesses
cause necrosis of the lung tissue involved and the subsequent
formation of a cavity, surrounded by consolidation. If this results
in communication with an airway, the cavity fills with air and
produces an airefluid level (the fluid represents necrotic debris,
see Figure 4).
Anaerobic bacteria, normally found as commensals within
the oropharynx and gastrointestinal tract, can cause abscess
formation.5 The bacteria gain entry to the lungs in aspirated
secretions or vomitus, leading to what is termed ‘aspiration
pneumonia’. This typically manifests as patchy consolidation
in the mid zones of the lungs, with or without cavitation (see
Figure 4).
S. aureus and Klebsiella pneumoniae (Gram negative) are two
more examples of bacteria that cause cavitation within areas
of consolidation, usually as part of a widespread
bronchopneumonia.1,2
TB frequently causes cavitation as well, predominantly within
the upper and mid zone opacities mentioned previously.
Secondary complications of pneumonia
Pleural effusion e common feature, correctly termed ‘para-
pneumonic’ effusion, occurs in association with many different
organisms. Should resolve with resolution of pneumonia.
Empyema e an abscess localized within the pleura, requires
prompt drainage. Usually occurs secondary to an effusion that
fails to resolve and often located in unusual locations on CXR,
e.g. effusion fluid localized along the lateral thoracic wall (as
opposed to the costophrenic angles).
Pneumatocele e air-filled cyst, resembles an abscess (air-fil-
led area surrounded by consolidation, see Figure 4) but more
spherical and without airefluid level. Classically associated
with Staphylococcus aureus and paediatric pneumonias.
Following resolution of pneumonia, pneumatoceles appears
as circular, air-filled areas surrounded by an extremely thin
opaque wall. This appearance is classically seen following
Pneumocystis pneumonia.
Pneumothorax e rare association with pneumonia, usually
secondary to ruptured pneumatocele or extension of air-filled
lung abscess into pleura.
Alternative causes of consolidation on CXR
Blood e especially in trauma settings or secondary to condi-
tions known to cause pulmonary haemorrhage, e.g. Good-
pasture’s syndrome. Typically causes patchy consolidation
and may be associated with rib fractures in trauma.
Oedema e causes an interstitial pattern in early stages, and
dense, air-space consolidation when severe. Classical features
include bilateral mid zone (‘peri-hilar’) consolidation, Kerly B
lines (fine reticulations at the lateral edges of the lungs), and
cardiomegaly (cardio-thoracic ratio >50%).
Non-infectious inflammation e includes adverse drug reac-
tions and idiopathic conditions, e.g. non-specific interstitial
pneumonia (NSIP). Produces a range of appearances ranging
from homogenous or patchy consolidation, to a reticulo-
nodular pattern.
Cancer e bronchoalveolar carcinoma can cause patchy
consolidation that does not resolve with antibiotics.
As the last example suggests, all pneumonias must be followed
up with repeat CXR during and/or following treatment. Failure to
respond to therapy, denoted primarily by a lack of clinical
improvement in the patient and persistence/progression of the
initial CXR appearances, can suggest an alternative diagnosis.
Remember:
consolidation takes up to 6 weeks to resolve on CXR and often
lags behind clinical resolution of the infection. A
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2 Herold CJ, Sailer JG. Community-acquired and nosocomial pneumonia.
Eur Radiol 2004; 14: E2e20.
3 Wilson AG. Interpreting the chest radiograph. In: Grainger RG,
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Figure 4 Lung abscess secondary to aspiration pneumonia. Note the
opacity (air efluid level) within the area of consolidation in the left mid
zone (black arrow).
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SURGERY 27:10 455 2009 Published by Elsevier Ltd.